The merged agency will trade as TRO and be led by TRO chief executive Rob Allen and his existing executive management team.
CPM Group chief operating officer Paul Wooley becomes non executive chairman, and CPM UK managing director Mike Hughes moves to a non-executive director role.
"We have found our best strategic partner," said TRO chief executive Rob Allen. "We are proud of the position we have reached as an independent agency, but it was clear that the best way to keep TRO fully primed to meet our clients future needs was to align ourselves with a partner that will help us achieve our goals."
TRO refused to comment on the cost of the deal.